The apparatus is of the kind comprising a light source directed towards the surface to be examined, a photodetector means sensitive to the light reflected by the surface, means to measure the specular reflection and the diffuse reflection from the surface and means to determine the brightness from the measurement of the specular reflection and of the diffuse reflection. On this subject, document FR-A-2 650 890 can be consulted. The tests carried out have shown that such an apparatus, while giving satisfactory results, has a relatively small sensitivity and discriminative power.
Document EP B 0 475 803 also discloses an apparatus designed to examine a surface, comprising a source of light capable of emitting a beam incident on the surface to be examined, means comprising a polarizer and at least one analyser making it possible to measure the reflection either with the directions of the polarizer and the analyser oriented in parallel, or oriented at right angles, the polarizer being placed between the light source and the surface, while the analyser is placed in the path of the reflected beam, photodetector means sensitive to the light reflected by the surface being, in addition, provided. The light source is directional and the polarized incident beam falls on the surface to be studied at an angle of incidence of between 0 and 90°, limits excluded, the direction of polarization of the incident beam being perpendicular to the plane of incidence. The apparatus is arranged in order to measure the reflection along at least two different reflection directions, one substantially symmetrical with the incident direction relative to the normal to the surface. The apparatus comprises means making it possible to differentiate, for each reflection direction, between the reflection in parallel polarization and analysis directions and the reflection with perpendicular polarization and analysis directions, the differences thus obtained forming a measurement of the so-called specular brightness and of the so-called diffuse brightness.
Such an apparatus functions properly, but only allows the examination of an elementary surface or of a point at a given instant.